acpi: cpuhp: spec: clarify 'CPU selector' register usage and endianness

acpi: cpuhp: spec: clarify 'CPU selector' register usage and endianness

* Move reserved registers to the top of the section, so reader would be
  aware of effects when reading registers description.
* State registers endianness explicitly at the beginning of the section
* Describe registers behavior in case of 'CPU selector' register contains
  value that doesn't point to a possible CPU.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <1575896942-331151-5-git-send-email-imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>